Molecular cloning and sequencing of genomic DNA encoding yeast vacuolar carboxypeptidase yscS

Vacuolar hydrolysis and efflux: current knowledge and unanswered questions

Hydrolysis within the vacuole in yeast and the lysosome in mammals is required for the degradation and recycling of a multitude of substrates, many of which are delivered to the vacuole/lysosome by autophagy. In humans, defects in lysosomal hydrolysis and efflux can have devastating consequences, and contribute to a class of diseases referred to as lysosomal storage disorders. Despite the importance of these processes, many of the proteins and regulatory mechanisms involved in hydrolysis and efflux are poorly understood. In this review, we describe our current knowledge of the vacuolar/lysosomal degradation and efflux of a vast array of substrates, focusing primarily on what is known in the yeast Saccharomyces cerevisiae. We also highlight many unanswered questions, the answers to which may lead to new advances in the treatment of lysosomal storage disorders. Abbreviations: Ams1: α-mannosidase; Ape1: aminopeptidase I; Ape3: aminopeptidase Y; Ape4: aspartyl aminopeptidase; Atg: autophagy related; Cps1: carboxypeptidase S; CTNS: cystinosin, lysosomal cystine transporter; CTSA: cathepsin A; CTSD: cathepsin D; Cvt: cytoplasm-to-vacuole targeting; Dap2: dipeptidyl aminopeptidase B; GS-bimane: glutathione-S-bimane; GSH: glutathione; LDs: lipid droplets; MVB: multivesicular body; PAS: phagophore assembly site; Pep4: proteinase A; PolyP: polyphosphate; Prb1: proteinase B; Prc1: carboxypeptidase Y; V-ATPase: vacuolar-type proton-translocating ATPase; VTC: vacuolar transporter chaperone.

Giardia duodenalis 14-3-3 protein is polyglycylated by a tubulin tyrosine ligase-like member and deglycylated by two metallocarboxypeptidases

The flagellated protozoan Giardia duodenalis is a parasite of the upper part of the small intestine of mammals, including humans, and an interesting biological model. Giardia harbors a single 14-3-3 isoform, a multifunctional protein family, that is modified at the C terminus by polyglycylation, an unusual post-translational modification consisting of the covalent addition of one or multiple glycines on the γ-carboxyl groups of specific glutamic acids. Polyglycylation affects the intracellular localization of g14-3-3, as the shortening of the polyglycine chain is correlated with a partial relocalization of 14-3-3 inside the nuclei during encystation. In this work we demonstrate that the gTTLL3, a member of the tubulin tyrosine ligase-like family, is the enzyme responsible for the 14-3-3 polyglycylation. We also identify two metallopeptidases of the M20 family, here termed gDIP1 (giardial dipeptidase 1) and gDIP2, as enzymes able to shorten the g14-3-3 polyglycine tail both in vivo and in vitro. Finally, we show that the ectopic expression of gDIP2 alters the g14-3-3 localization and strongly hampers the cyst formation. In conclusion, we have identified a polyglycylase and two deglycylases that act in concert to modulate the stage-dependent glycylation status of the multifunctional regulatory g14-3-3 protein in G. duodenalis.

Nitrogen catabolite repression in Saccharomyces cerevisiae

In Saccharomyces cerevisiae the expression of all known nitrogen catabolite pathways are regulated by four regulators known as Gln3, Gat1, Dal80, and Deh1. This is known as nitrogen catabolite repression (NCR). They bind to motifs in the promoter region to the consensus sequence 5'GATAA 3'. Gln3 and Gat1 act positively on gene expression whereas Dal80 and Deh1 act negatively. Expression of nitrogen catabolite pathway genes known to be regulated by these four regulators are glutamine, glutamate, proline, urea, arginine. GABA, and allantonie. In addition, the expression of the genes encoding the general amino acid permease and the ammonium permease are also regulated by these four regulatory proteins. Another group of genes whose expression is also regulated by Gln3, Gat1, Dal80, and Deh1 are some proteases, CPS1, PRB1, LAP1, and PEP4, responsible for the degradation of proteins into amino acids thereby providing a nitrogen source to the cell. In this review, all known promoter sequences related to expression of nitrogen catabolite pathways are discussed as well as other regulatory proteins. Overview of metabolic pathways and promotors are presented.

Nitrogen GATA factors participate in transcriptional regulation of vacuolar protease genes in Saccharomyces cerevisiae

The expression of most nitrogen catabolic genes in Saccharomyces cerevisiae is regulated at the level of transcription in response to the quality of nitrogen source available. This regulation is accomplished through four GATA-family transcription factors: two positively acting factors capable of transcriptional activation (Gln3p and Gat1p) and two negatively acting factors capable of down-regulating Gln3p- and/or Gat1p-dependent transcription (Dal80p and Deh1p). Current understanding of nitrogen-responsive transcriptional regulation is the result of extensive analysis of genes required for the catabolism of small molecules, e.g., amino acids, allantoin, or ammonia. However, cells contain another, equally important source of nitrogen, intracellular protein, which undergoes rapid turnover during special circumstances such as entry into stationary phase, and during sporulation. Here we show that the expression of some (CPS1, PEP4, PRB1, and LAP4) but not all (PRC1) vacuolar protease genes is nitrogen catabolite repression sensitive and is regulated by the GATA-family proteins Gln3p, Gat1p, and Dal80p. These observations extend the global participation of GATA-family transcription factors to include not only well-studied genes associated with the catabolism of small nitrogenous compounds but also genes whose products are responsible for the turnover of intracellular macromolecules. They also point to the usefulness of considering control of the nitrogen-responsive GATA factors when studying the regulation of the protein turnover machinery.

Stationary-phase gene expression in Saccharomyces cerevisiae during wine fermentation

Genetic engineering of wine yeast strains requires the identification of gene promoters specifically activated under wine processing conditions. In this study, transcriptional activation of specific genes was followed during the time course of wine fermentation by quantifying mRNA levels in a haploid wine strain of Saccharomyces cerevisiae grown on synthetic or natural winery musts. Northern analyses were performed using radioactive probes from 19 genes previously described as being expressed under laboratory growth conditions or on molasses in S. cerevisiae during the stationary phase and/or under nitrogen starvation. Nine genes, including members of the HSP family, showed a transition-phase induction profile. For three of them, mRNA transcripts could be detected until the end of the fermentation. Expression of one of these genes, HSP30, was further studied using a HSP30::lacZ fusion on both multicopy and monocopy expression vectors. The production of beta-galactosidase by recombinant cells was measured during cell growth and fermentation on synthetic and natural winery musts. We showed that the HSP30 promoter can induce high gene expression during late stationary phase and remains active until the end of the wine fermentation process. Similar expression profiles were obtained on five natural winery musts.

Analysis of the DNA sequence of a 15,500 bp fragment near the left telomere of chromosome XV from Saccharomyces cerevisiae reveals a putative sugar transporter, a carboxypeptidase homologue and two new open reading frames

We report the sequence of a 15.5 kb DNA segment located near the left telomere of chromosome XV of Saccharomyces cerevisiae. The sequence contains nine open reading frames (ORFs) longer than 300 bp. Three of them are internal to other ones. One corresponds to the gene LGT3 that encodes a putative sugar transporter. Three adjacent ORFs were separated by two stop codons in frame. These ORFs presented homology with the gene CPS1 that encodes carboxypeptidase S. The stop codons were not found in the same sequence derived from another yeast strain. Two other ORFs without significant homology in databases were also found. One of them, O0420, is very rich in serine and threonine and presents a series of repeated or similar amino acid stretches along the sequence.

Regulation of the yeast vacuolar aminopeptidase Y gene (APY1) expression

The steady-state levels of the aminopeptidase Y (APY1) transcript and the level of assayable aminopeptidase Y activity were measured under a variety of nutritional conditions. Specific APY1 mRNA is less abundant in cells grown in minimal medium than in cells grown in rich medium, while active enzyme levels follow just the opposite pattern. Aminopeptidase Y activity decreases when cells are deprived of glucose without a concomitant fall in mRNA levels. Production of aminopeptidase Y is not markedly affected by nitrogen regulation. APY1 gene expression is not disturbed in heat-shocked cells. Our data support the idea that the main control event in vacuolar aminopeptidase Y expression is a post-transcriptional step.

Review: biosynthesis and function of yeast vacuolar proteases

The yeast vacuole, which is equivalent to the lysosome of higher eukaryotes, is one of the best characterized degradative organelles. This review describes the biosynthesis and function of yeast vacuolar proteases. Most of these enzymes are delivered to the vacuole via the early compartments of the secretory pathway and the endosome, while one of them is directly imported from the cytoplasm. The proteases are synthesized as precursors which undergo many post-translational modifications before the final active form is generated. Proteolytic activation by developments in the analysis of the functions of vacuolar proteolysis are described. Substrates of the vacuolar proteases are mostly imported via endocytosis or autophagocytosis, and vacuolar proteolysis appears to be mainly important under nutritional stress conditions and sporulation.

Cis and trans-acting regulatory elements required for regulation of the CPS1 gene in Saccharomyces cerevisiae

To clarify the transcriptional regulation by nutrient limitation of the gene encoding carboxypeptidase yscS in Saccharomyces cerevisiae (CPS1), we performed an analysis of its 5' noncoding region. In deletion experiments a sequence located between positions -644 and -591 was found to be responsible for transcriptional repression of the CPS1 gene in yeast cells grown on rich nitrogen sources. Furthermore, a 162 bp fragment spanning positions -644 to -482 of the promoter of the CPS1 gene repressed gene expression when placed 3' to the upstream activation sequence (UAS) of the heterologous gene CYC1. A fragment containing this putative upstream repression sequence (URS) was shown specifically to bind protein from a yeast extract as demonstrated by gel retardation experiments. Although a sequence mediating the control of gene expression by GCN4 was found within the URS element, the GCN4 gene product is not required for DNA-binding activity. In addition, at least three other upstream activation UASs responsible for the activation of CPS1 expression by glucose under nitrogen starvation conditions were found to be located between positions -673 and -644, -482 and -353, and -243 and -186, respectively. The putative mechanism of the nitrogen limitation-dependent regulation of CPS1 expression via these regulatory elements is discussed.

Control of Saccharomyces cerevisiae carboxypeptidase S (CPS1) gene expression under nutrient limitation

Expression of the vacuolar carboxypeptidase S (CPS1) gene in Saccharomyces cerevisiae is regulated by the availability of nutrients. Enzyme production is sensitive to nitrogen catabolite repression; i.e. the presence of ammonium ions maintains expression of the gene at a low level. Transfer of ammonium-glucose pre-grown cells to a medium deprived of nitrogen causes a drastic increase in CPS1 RNA level provided that a readily usable carbon source, such as glucose or fructose, is available to the cells. Derepression of the gene by nitrogen limitation is cycloheximide-insensitive. Neither glycerol, ethanol, acetate nor galactose support derepression of CPS1 expression under nitrogen starvation conditions. Non-metabolizable sugar analogs (2-deoxyglucose, 6-methyl-glucose or glucosamine) do not allow derepression of CPS1, showing that the process is energy-dependent. Production of carboxypeptidase yscS also increases several-fold when ammonium-pregrown cells are transferred to media containing glucose and a non-readily metabolizable nitrogen source such as proline, leucine, valine or leucyl-glycine. Analysis of CPS1 expression in RAS2+ (high cAMP) and ras2 mutant (low cAMP) strains and in cells grown at low temperature (23 degrees C) and in heat-shocked cells (38 degrees C) shows that steady-state levels of CPS1 mRNA are not controlled by a low cAMP level-signalling pathway.

Molecular cloning of soluble aminopeptidases from Saccharomyces cerevisiae. Sequence analysis of aminopeptidase yscII, a putative zinc-metallopeptidase

Plasmids capable of complementing lap1, lap2 and lap3 mutations [R.J. Trumbly and G. Bradley (1983) J. Bacteriol. 156, 36-48] were isolated from a yeast YEp13 library by screening for activity against the chromogenic aminopeptidase substrate L-leucine beta-naphthylamide in intact yeast colonies. The genomic inserts were shown to contain the structural genes for aminopeptidases yscII, yscIII and yscIV. Plasmids containing the gene encoding aminopeptidase yscII of Saccharomyces cerevisiae, APE2 (LAP1) were analyzed in detail. APE2 was determined by DNA blot analysis to be a single-copy gene located on chromosome XI. The cloned fragment was used to identify a 2.7-kb mRNA. The cloned APE2 gene was sequenced and found to consist of an open reading frame of 2583 bp encoding a protein of 861 amino acids. The protein sequence contains two putative N-glycosylation sites. A significant amino acid similarity was detected between the APE2 gene product and members of the zinc-dependent metallopeptidase gene family. Chromosomal disruption of the APE2 gene completely abolishes the distinct activity band previously identified as aminopeptidase yscII [H.H. Hirsch, P. Suárez-Rendueles, T. Achstetter and D.H. Wolf (1988) Eur. J. Biochem. 173, 589-598] in crude extracts subjected to non-denaturing polyacrylamide gel electrophoresis and subsequent aminopeptidase activity staining. No vital consequence of aminopeptidase yscII absence on cell growth could be detected.